Subject Test Biology

Question 5 of 5

Thymine is used by animal cells primarily for the synthesis of DNA. A group of sea urchin eggs was fertilized in sea water containing radioactive thymine. Following fertilization, samples of embryos were removed at regular intervals and the radioactivity in the nucleic acid was measured in counts per minute. The results obtained are shown in the figure below.

An appropriate control to show that this experiment measures DNA synthesis and not RNA synthesis would be to perform the same procedures but

Select an Answer

not fertilize the eggs

Correct Answer:

No

sample the embryos at longer time intervals

Correct Answer:

No

add radioactive uracil instead of radioactive thymine

Correct Answer:

Yes

fertilize the eggs in sea water that does not contain radioactive thymine

Correct Answer:

No

count the number of cells in the embryos at the beginning and at the end of the experiment

RNA contains uracil instead of thymine. Therefore, in order to test whether RNA synthesis occurred — and to prove that this experiment measures DNA synthesis and not RNA synthesis — the same procedures would need to be repeated with radioactive uracil substituted for radioactive thymine.